NU OCT (Nu Octantis). As if fourth magnitude (3.76) Nu Octantis, the luminary of Octans (the Octant, one of the sky's several navigational instruments), is not already obscure enough for being in such a dim constellation, it sinks further into obscurity by being quite overwhelmed by the fame of the constellation's best-known star, the fifth magnitude (5.47, nearly sixth) southern pole star, Sigma Octantis. Way down the Greek letter list, Nu Oct oddly beats out the next star down, fifth magnitude (4.15) Beta Oct, while the Alpha star, usually a constellation's brightest, is a full magnitude (5.15) fainter yet. In spite of its seeming anonymity, which is enhanced by being yet one more yellow-orange class K (K0) giant, Nu Oct harbors a pleasant surprise by providing us with a wonderful chance to see what is going to happen to the Sun. For a giant, its character is modest indeed. As befits this humble star, there is no temperature measure, so 4700 Kelvin must be adopted from its K-giant class. From its distance of 69 light years (at least we have that much), the apparent magnitude, and the amount of infrared radiation implied by temperature, we calculate a luminosity only 17 times that of the Sun, making Nu Oct one of the least radiant of celestial giants. Consistently, Nu Oct is relatively small, its remarkably low radius of 6.3 times solar hardly befitting a star called a "giant." The reasons involve mass and evolutionary status. From the theory of stellar structure and evolution, Nu Octantis weighs in at almost exactly one solar mass (perhaps just a bit more). After giving up hydrogen fusion in its core some two billion years ago, it began to swell some and to cool as a subgiant. It is now in the process of expanding much more vigorously and brightening. In a sense we are watching the "birth" of a genuine giant, one that in "just" 100 million years or so will become more than 60 times brighter and 15 times larger than it is today (before it settles down to fuse its helium into carbon and oxygen). Assuming a solar mass, the star's current age would come in at 12.1 billion years (less if greater mass). Given that the Sun has a well-determined age (from radioactive dating of meteorites) of 4.6 billion years, Nu Oct provides a brilliant lesson in what our Sun will look like in another 7.5 billion. But in Nu Octantis's case, we probably need not worry about what is going to happen to its planets, as the star has a binary companion close enough to it to make a planetary system unlikely (though given the odd planetary systems astronomers are finding, who knows?). The dim companion, about which nothing is known, orbits with a period of 2.8 years and lies somewhere between one and two Astronomical Units from Nu Oct proper (that is, within the orbital radii of Earth and Mars). The binarity, however, does not preclude the star's real story. If you can't yourself wait another 7.5 billion years to follow the solar tale, look to the luminary of the Octant.

UPDATE. Nu Octantis has a planet! Maybe. And if so, it's a real oddity. Intense study, including satellite observation, confirms most of the above conclusions. Nu Oct is a spectroscopic binary that consists of a K1 giant with a temperature of 4790 Kelvin, a luminosity of 16 Suns, a radius of 5.9 solar, and a mass of 1.4 Suns in mutual 2.9-year orbit with a K7-M1 dwarf with a mass of 0.5 Suns. The orbital axis is tilted 71 degrees to the line of sight. The mean separation is 2.55 Astronomical Units, with a modest eccentricity taking the pair between 2 and 3 AU. Disturbances in the larger star's (Nu Oct A's) spectrum suggest a planet orbiting it at 1.2 AU with a mass at least 2.5 jupiters. The two "resonate," such that the planet goes around twice for every five of Nu Oct B's orbits. Such a planet would have a highly unstable orbit and it is hard to see how it could exist (unlike the case for 16 Cygni B, where the two stars are well-separated). However, theory shows that if the planet went around backwards (relative to Nu Oct B's motion), it could survive being ejected. There are other possibilities for the spectral disturbances, and the reality of the planet has yet to be confirmed. If real, the planet has two Suns, one dying, the other feeble, the luminasry of Octans thus adding to our knowledge of the variety of planetary systems out there. (Data and information from a papers in the (1)Astrophysical Journal by J. Eberle and M. Cuntz and (2) Monthly Notices of the Royal Astronomical Society by D. J. Ramm, D. Pourbaix, J. B. Hearnshaw, and S. Komonjinda.)
Original written by Jim Kaler 10/13/06; updated 10/22/10. Return to STARS.